The present invention relates to a cutting insert for metal drilling. The insert is intended to be secured into a drill body that can be equipped with several identically shaped inserts. The drill body is normally provided with chip flutes, an inner insert site disposed close to the drill center and an outer insert site disposed close to the periphery. This type of drill has cylindrical basic shape defined by a diameter and is rotatable around a central axis.
A drill of this type is disclosed in EP-A-1 81 844 (corresponding to Karlsson et al. U.S. Pat. No. 4,648,760). The inserts therein are basically of rectangular shape where the short end surfaces are confined by two short cutting edges which intersect one another at an obtuse angle, wherein the radially outer insert is located such that it defines the diameter of the drilled hole. The longitudinal axis of the radially inner insert extends parallel with the center axis of the drill whilst overlapping the same.
When providing drills of the aforementioned type it is often difficult to achieve desirable small diameters and at the same time achieve desirable surface finish when drilling in metallic workpieces. Thus, a primary object of the present invention is to provide a new type of cutting insert for drills that enables an improved surface finish to be achieved when drilling in metal. Another object of the present invention is to provide a new type of insert for drills with such optimized geometry that improved xe2x80x9cfeelxe2x80x9d is possible during drilling without simultaneously impaired surface finish of the hole wall in the hole to be drilled.
These and other objects are achieved by the insert of the present invention, which comprises a body having a polygonal shape with substantially flat upper and lower surfaces, and edge surfaces adjoining the upper and lower surfaces. The lower surface has a smaller area than the upper surface to provide the insert with a positive cutting geometry. A first of the edge surfaces intersects the upper surface to form a main cutting edge. The main cutting edge comprises short cutting edge portions converging from respective ends of the cutting edge and forming an obtuse angle with one another. An end of the first edge surface joins a second one of the edge surfaces by a curved nose portion. The curved nose portion intersects the upper surface to form a curved cutting edge defined by a plurality of sequentially arranged curved cutting edge segments having respective radii. The radii have different respective lengths.